CYTOGENETICS

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CYTOGENETICS
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Terminology for Sets of Chromosomes
Euploid normal chromosome sets Aneuploid
incomplete (unbalanced) chromosome sets In humans
-- aneuploidy in up to 35 of spontaneous
abortions (6-20 weeks)
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Types of Aneuploidy
Monosomy 2n - 1 Human (females) -- only one kind
of monosomy 1 in 20,000 live births Trisomy 2n
1 Most common (at conception ?) -- chr 16 Most
common at live birth -- trisomy 21 Down syndrome
-- 1 in 750 live births Less common trisomy 18
(1 in 10,000) trisomy 13 (1 in 20,000) Why
better survival with trisomy 21 than other
trisomies?
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• Hierarchy of tolerance of aneuploidy
• sex chromosome aneuploidy gt autosomal aneuploid
• eg. XXY gt monosomy, trisomy
• autosomal trisomy gt monosomy

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Major cause of aneuploidy nondisjunction during
meiosis

• Consequences
• Defective products
• Heterogeneous allele composition

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Aneuploidy and maternal age
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About 20-25 of Down syndrome cases are due to
paternal nondisjunction Sometimes, the pedigree
can provide clues as to whether aneuploidy arose
from maternal or paternal nondisjunction...
Xg X-linked recessive condition Paternal or
maternal ND here?
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Mitotic nondisjunction e.g., Down syndrome
mosaics
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Deletions

• Terminal vs. Interstitial
• Cri du chat" syndrome in humans -- terminal
  deletion in Chromosome 5
• How are these deletion chromosomes transmitted?

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Genetic consequences

• Reduced recombination frequency between markers
  flanking the deletion

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Practical use deletion mapping to locate
genes Set up crosses such that the progeny have
the recessive allele of interest on one homolog
and a deletion on the other Which deletion
uncovers the recessive phenotype?
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Duplications
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Anticipation Progressively earlier onset
Mechanism of disease? Mechanism of expansion?
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Translocations

• Often reciprocal
• Double heterozygotes can be viable
• Phenotypes
• Associated with specific forms of cancer
• e.g., Burkitt lymphoma
• one partner chromosome 8
• other partner chromosome 14, 22, or 2

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• Phenotypes
• Non-cancer disorders
• e.g., Translocation Down syndrome
• Robertsonian translocation between chr 14 and 21
• long arms of two acrocentric chromosomes fused

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Pairing and meiosis in double heterozygotes
Consequences Semi-sterility
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Table of Karyotypes and Spreads (normal humans)
Chromosome spread with chromosomes shown by
bright field G-banding
Karyotype shown by bright field G-banding of
chromosomes
Idiogram of G-banded chromosome
Same as for male (but without the Y)
http//www.pathology.washington.edu/Cytogallery/cy
togallery.html
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Abnormalities detected by cytogenetics
Translocations
Deletions
http//www.waisman.wisc.edu/cytogenetics/abnormali
ties/abnormalities.html
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Karyotype Preparation from Arrested
Metaphase (Chromosomes Metaphase Chromosomes)
1. Obtaining Cells for Chromosome Study
a. Amniocentesis b. Chorionic Villus
Sampling c. Fetal Cell Sorting 2.
Preparing cells for Chromosome Observation
a. Cells are grown in tissue culture b.
Spindle inhibitor added to the culture to stop
cells in Metaphase c. Slide prepared
Swelling, Squashing, and Staining d. Cell
with a good set of chromosomes is Found
e. Chromosomes arranged in sets of Homologous
pairs 3. Karyotype preparation a.
Chromosomes cut out, Arranged in sets b.
By size from largest to smallest c. By
the position of centromere (metacentric,
acrocentric, submetacentric)
http//www.st-elizabeth.edu/_ikessler/hgen/genlec5
.html
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Chromosomal Abnormalities of the Sex Chromosomes

• Turner's syndrome XO, 2N-1 (13500 females)
• 1 of 2500 (uncommon, monosomic more lethal than
  trisomic)
• May be mosaic, mix of 2N 2N-1 cells
• Appears normal, grows slowly
• Normal IQ, weakness in math, Faulty spatial
  perception
• Shield chest when young
• At birth (may) Flairing of Neck (Webbing)
• Abnormally Low hair line, widely spaced nipples
• Secondary sex characteristics do NOT develop
  normally
• 1.fails to menstruate
• 2.Infertile, ovarian dysgenesis (no
  eggs)
• Estrogen replacement for Secondary sex
  development
• Life threatening Cardiac Renal disease

http//www.st-elizabeth.edu/_ikessler/hgen/genlec5
.html
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Chromosomal Abnormalities of the Sex Chromosomes

• Klienfelters Syndrome XXY, may be mosaic
• 1 of 500 males
• Taller than average, partial breast development
• Secondary sex characteristics do NOT develop
  normally
• Small testicles, high pitched voice, female hair
  distribution
• Altered body proportions, hips slighly larger
  than normal male
• Appears normal, marries but can not have
  children
• 5 of males in fertility clinics
• IQ about 90
• Cause maternal non-disjunction

http//www.st-elizabeth.edu/_ikessler/hgen/genlec5
.html
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Natural Abortions Sex Chromosome Abnormalities
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Chromosome 1
Cytogenetic Clinical entities 1p32
rearrangements 1q rearrangements in multiple
myeloma t(12)(q25p23) (go to next
slide) t(13)(p32p21) t(13)(p36q21) t(15)(p32
q31) t(17)(p32q34) t(17)(p36q34) t(17)(q10p1
0) t(111)(q21q23) t(112)(q25p13) t(114)(p22q
32) in non Hodgkin¹s lymphoma (NHL) t(114)(p32q1
1) t(118)(q25q23) t(119)(q23p13) t(121)(p36q
22) t(122)(p13q13)


Genes Symbol
Location ABL2 1q25 AF1p
1p32 AF1q
1q21 N-RAS 1p13 PAGA
1p34.1 PBX1
1q23 PRCC 1q21.2 PSF
1p34 TAL1
1p32 RAP1A 1p13.3 TPM3
1q25 OTT
1p13
http//www.infobiogen.fr/services/chromcancer/Inde
xbychrom/idx_1.html
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T(12)(q25p23) Information
Clinics and Pathology Disease
Inflammatory myofibroblastic tumors Clinics
Rare soft tissue tumour found in children
and young adults Pathology Spindle
cell proliferation with myofibroblastic
differenciation and an inflammatory
infiltrate Prognosis Low malignant
potential and good prognosis Genes involved and
Proteins Gene Name TPM3 (tropomyosin alpha
chain) Location 1q25 Protein
284 amino acids, 33 kDa coiled coil
structure role in Calcium dependant
actin-myosin interaction Gene Name ALK
Location 2p23 Protein 1620
amino acids 177 kDa glycoprotein (200 kDa
mature protein) membrane associated
tyrosine kinase receptor Result of the
chromosomal anomaly Hybrid gene Description
5' TPM3 - 3' ALK
http//www.infobiogen.fr/services/chromcancer/Anom
alies/t0102ID1168.html